1. Field of the Invention
This invention relates to a process for manufacturing a magnetic recording medium, and more particularly to a process for manufacturing a magnetic recording medium wherein a magnetic thin film is formed by a multicomponent simultaneous vapor deposition process.
2. Description of the Prior art
For the manufacture of a conventional magnetic recording medium, a process using a particulate coating medium has been widely employed. This type is manufactured by a process in which acicular-shaped magnetic powders and an organic binder are mixed together in an organic solvent to produce a magnetic paint, or coating, including the magnetic powders well dispersed therein, and this paint is applied to a non-magnetic substrate to form a coated film and, following magnetic orientation, the coated film is dried.
More recently, there may be mentioned a ferromagnetic metal thin film medium which is being put to practical use to comply with a demand for high density recordings. This type is manufactured by a process in which a ferro-magnetic metal thin film is formed on a non-magnetic substrate by a wet plating technique or a physical vapor deposition technique such as vapor deposition, ion plating, sputtering, etc. Typical examples of this recording medium are a magnetic recording medium for longitudinal mode magnetic recording comprising a magnetic layer composed essentially of a ferromagnetic metal such as Co, Fe, Ni, etc., and a magnetic recording medium for perpendicular mode magnetic recording comprising a magnetic layer composed essentially of a Co-Cr alloy.
Further, for the purpose of improving this ferromagnetic metal thin film recording medium, there has been reported a process in which an organic polymer is deposited on the magnetic thin film thereof for forming a protective layer or a lubricating layer.
Furthermore, there has been reported a process in which a ferromagnetic substance and an organic polymer or an organic lubricant are simultaneously deposited on a support by a simultaneous vapor deposition process to form a magnetic recording layer (U.S. Pat. No. 4,260,466 and U.S. Pat. No. 4,476,195).
With respect to the particulate coating magnetic recording medium which has been widely employed, improved performance thereof has been made possible by development of acicular-shaped magnetic powders, also in response to the demand for higher recording density in recent years. However, this technique is defective in that it is difficult to make the thickness of the magnetic layer thin, because a decrease in the thickness thereof is limited in terms of saturation magnetic flux density and reproduced output. Thus, the improvement in the recording density thereof is limited. It is clear, therefore, that this type is not suitable for a high density magnetic recording medium.
Additionally, the particulate type magnetic recording medium involves such disadvantages that the manufacturing process thereof is complicated, and manufacturing facilities and associated facilities such as apparatus for treatment of organic solvent, magnetic powders and organic chemicals or the like, which are essential for the manufacturing process and for recovery of the organic solvent, are extensive and require high costs.
With respect to the ferromagnetic metal thin film medium typically represented by a magnetic tape prepared by a vapor deposition process, because a non-magnetic material is not contained in the magnetic layer thereof, the saturation magnetic flux density can be increased. Also, because it is possible to decrease the thickness of the magnetic layer, which is necessary for high density recording, increasing of the recording density can be achieved.
However, this type of magnetic recording medium is defective in that the adhesion strength of the ferromagnetic thin film to the non-magnetic substrate is so weak that, during running of the medium at a high speed, the magnetic film layer is liable to suffer mechanical deterioration such as damage, peeling or the like caused by the running system such as a magnetic head, a drum or the like. Additionally, this type of magnetic recording medium is liable to suffer from corrosion or the like, because the magnetic layer thereof is inferior in environmental stability. Thus, this type of magnetic recording medium is difficult to put to practical use as a recording medium because it is defective in mechanical durability and chemical corrosion resistance as mentioned above.
With respect to the foregoing process for improving the foregoing defects, an organic polymer is deposited on the magnetic thin film to form a protective layer. The resultant magnetic medium is defective, however, in that there is a spacing loss caused in the course of recording and reproduction by the protecting layer which has a comparatively large thickness. Degradation in recording and reproduction results.
Regarding the foregoing process for manufacturing a magnetic recording medium for longitudinal mode recording by a simultaneous vapor deposition process as disclosed in the foregoing U.S. patents, the resultant composite film of the ferromagnetic substance and the polymer is defective in that it contributes little to a shape magnetic anisotropy resulted from its film structure. Consequently, the in-plane magnetic anisotropy is weak, and also the squareness ratio and the coercive force in the longitudinal direction are small.
These defects result in a lowering of reproduced output and sensitivity, and thus the product is not suitable for practical use as a magnetic recording medium. Through investigation of the cause of these defects it has been determined according to the present invention that the defects result from the fact that the organic polymer is deposited on the substrate at a normal incidence or at an oblique incidence of a low incident angle.
In the course of the research to reach the present invention, it has been found that an incident angle of an incident beam of polymer to a substrate is important. Namely, in manufacturing a magnetic recording medium, it is essential that an incident beam of polymer is irradiated on the surface of the substrate by an oblique incidence of high incident angle. This gives a large magnetic anisotropy to a composite film, and makes it possible to obtain a thin film medium having a useful and practical magnetic property.
Thus, by the foregoing conventional processes, it has been impossible to provide a magnetic recording medium which has a fully satisfactory high magnetic property, in addition to improved mechanical durability and improved chemical corrosion resistance. As mentioned above, even by a conventional simultaneous vapor deposition process, the resultant composite film of the ferromagnetic substance and the polymer is such that it contributes only little to a shape magnetic anisotropy, and is small in coercive force and squareness ratio. Consequently, such a magnetic recording medium as above is not sufficient in reproduced output and sensitivity, and is not of practical use.